1. Field of the Invention
The present invention relates to a process for the conversion of light hydrocarbons (HCs) through the use of plasma and, in particular, through gliding electric discharges in the presence of oxygen O.sub.2 and possibly of water vapor H.sub.2 O added to these HCs.
2. Description of the Relevant Art
The production of syngas from light saturated HCs is a relatively well known and very important stage, especially for a chemical valorization of the natural gasses (NGs). A chemical valorization of immense resources of NG would be much more interesting than its total finite combustion towards the direct recovery of energy in the furnaces, boilers or turbines. There are also situations where the NGs with high HC content are released to the earth's atmosphere, even without any recovery of energy; this may be illustrated by oil field flares that burn a hydrocarbon contained gas known as "associated gas", or by different emanations of biogas (mixture mainly of methane and carbon dioxide of near equal content) originating from an anaerobic fermentation of organic waste. Any emission of needlessly burned and, especially, unburned HC contributes heavily to the atmospheric pollution.
The most used current process to produce syngas, the catalytic steam reforming process (or also "steam reforming"), encounters major difficulties. In principle, it requires only a high temperature (thermodynamic reason) and a high pressure (kinetic reason). However, in practice, in spite of the know-how for the production of syngas according to this process, the joint control of the compositions, pressures and temperatures is delicate, even impossible, without resorting to the use of catalysts. Therefore, in order to perform the steam reforming of a NG, a catalytic device is generally selected: presence of solid material under highly dispersed and active form (with a specific surface of at least one hundred m.sup.2 per gram) for temperatures that may be reached without great difficulty. The classic steam reforming technology thus used requires furnaces containing several hundreds of brittle metal pipes (filled with a catalyst and of which the length may reach several tens of meters), heated externally with NG.
Thus, the large quantities of carbon dioxide originating from the combustion are discharged into the atmosphere by these furnaces, which have a very poor thermal performance. This technology is also linked to very high pressure losses. The temperature that may be withstood by the pipes also prevents the reduction of the CO.sub.2 content in the actual syngas (hindrance product resulting from a side reaction at a temperature too low). Other difficulties are related to the contamination of the catalysts by sulfur and/or nitrogen, the aging of the catalysts, the necessary excess of steam and/or the formation of soots (which block the entire tubular system at the macroscopic scale and, in particular, the microscopic pores of the catalyst). These difficulties are observed mainly in the steam reforming of HCs heavier than methane; they are more brittle and, thus, more coking.
A bibliographic research covering the last three decades gave very few published results regarding the partially oxidizing conversion of saturated HCs assisted by oxygen and plasma. This might be due to difficulties linked to the presence of free oxygen attacking the conventional tungsten or graphite electrodes of traditional plasma devices.
We know of only one attempt to use plasma in this environment (outside of our own efforts). P. CAPEZZUTO et al. ["The oxidation of methane with carbon dioxide, water vapor, and oxygen in radio-frequency discharges at moderate pressures", 3.sup.rd Int. Symp. on Plasma Chemistry, Limoges, 1976, contribution G.5.11, 7 pp.] studied a partial oxidation of methane mixed separately with CO.sub.2, either with O.sub.2 or H2O, with oxidant/CH.sub.4 molar ratio=1. The 35 MHz RF plasma reactor needed an additional flux of argon and could only work at a very low pressure of approximately 2.7 kPa. For a total incoming gas flow of 3 to 36 l(n)/min, the energy density was very high and ranged from 1 to 12 kWh/m.sup.3. No industrialization was possible due to the high electric power and noble gas consumption (in addition to the complicated electric power supply and the need to work under vacuum). Due to the mechanical requirements of implantation, the poor energy performance and the insufficient unit powers of RF plasma sources, this method is poorly adapted, from an economic point of view, for the transformation of significant gas flows.
In Orleans, since 1986, we also worked on the conversion of NGs in thermal plasma reactors. These classic simple or transferred arc plasma torches make it possible to obtain small volume plasmas at very high temperatures (T&gt;10 kK). Although these devices are potential sources of active species, they are however poorly adapted to the chemistry applications that require much lower temperatures (in order not to destroy the hydrocarbon molecules to the extent that they become soot) and especially larger volumes, filled with plasma, in order to act closely on the entire fluid to b e processed. The plasma torch technology that is, for example, well established in the solid projection field, was found to be both costly and very difficult to implement for chemical processes.
However, we achieved improvements in the field of thermal plasmas, in the case of a transformation of methane in a specifically controlled electric arc, see P. JORGENSEN et al., "Process of Production of Reactive Gases Rich in Hydrogen and Carbon Monoxide in an Electric Post-Arc", BF 2 593 493 (1986). The structure of the device, as implemented then, did unfortunately not make it possible to use water vapor as reactant or to work without consuming the argon necessary as gas forming the plasma of a first pilot arc. We then used almost the same high current (20-150 A) arc to study the oxidation of ethane, see K. MEGUERNES et al., "Oxidation of ethane C.sub.2 H.sub.6 by CO.sub.2 or O.sub.2 in an electric arc", J. High Temp. Chem. Process, vol. 1(3), p. 71-76 (1992) without much improvement in the consumption of electric power and argon.